Fiber reinforced polymer (FRP) - confined concrete-steel composite columns have been recently introduced as a retrofit technique for structural steel columns. This technique involves placing an FRP tube around an in situ steel column and subsequently filling the void between the steel section and the FRP tube with concrete to create a composite column. The composite action occurs due to the concrete encasing the steel section and the FRP confining the concrete. An experimental investigation has been undertaken to evaluate: the effect of adding a steel W section to confined concrete; the effect of using a split tube system as a practical application technique; and the effect of shrinkage reducing admixture in confined concrete. A total of eighteen stub columns, consisting of six different column types were tested to meet these objectives. It was found that the load-displacement response of confined concrete that includes a steel W section is similar to the response of confined concrete plus the elastic-perfectly plastic steel contribution. Experimental findings demonstrate the enhanced performance of the proposed split tube retrofit technique. An analytical model was used to predict the response of the different column types. A method is proposed to incorporate the increased stiffness of the FRP jacket in the split tube system and is shown to be an improvement on the original model. A parametric study was also performed on the revised model.